Advertisements
Advertisements
Question
Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?
Solution
Potassium dichromate is prepared from chromate, which in turn is obtained by the fusion of chromite ore (FeCr2O3) with sodium or potassium carbonate in free excess of air. The reaction with sodium carbonate occurs as follows:
\[\ce{4FeCr2O4 + 8Na2CO3 + 7O2 -> 8Na2CrO4 + 2Fe2O3 + 8CO2}\]
The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na2Cr2O7.2H2O can be crystallised.
\[\ce{2Na2CrO4 + 2H+ -> Na2Cr2O7 + 2Na+ + H2O}\]
Sodium dichromate is more soluble than potassium dichromate. The latter is therefore prepared by treating the solution of sodium dichromate with potassium chloride.
\[\ce{Na2Cr2O7 + 2KCl -> K2Cr2O7 + 2NaCl}\]
Orange crystals of potassium dichromate crystallise out. The chromates and dichromates are interconvertible in an aqueous solution depending upon the pH of the solution. The oxidation state of chromium in chromate and dichromate is the same.
\[\ce{2CrO^2-_4 + 2H+ -> Cr2O^2-_7 + H2O}\]
\[\ce{Cr2O^2-_7 + 2OH- -> 2CrO^2-_4 + H2O}\]
APPEARS IN
RELATED QUESTIONS
Complete the following equation:
\[\ce{2MnO4- + 6H+ + 5NO2- ->}\]
Complete the following chemical equation
8MnO4- + 3S2O32- + H2O →
Complete the following equations : 2 Na2CrO4 + 2 H + →
Give an example and suggest a reason for the following feature of the transition metal chemistry:
A transition metal exhibits the highest oxidation state in oxides and fluorides.
Indicate the steps in the preparation of KMnO4 from pyrolusite ore.
Write the formula of an oxo-anion of Manganese (Mn) in which it shows the oxidation state equal to its group number.
Name a member of the lanthanoid series that is well-known to exhibit +2 oxidation state.
E0 value for Mn3+ Mn2+ couple is much more positive than that for Cr3-/ Cr2-.
Write the chemical equation for the preparation of 4 KMnO from 2. MnO Why does purple colour of acidified permanganate solution decolourise when it oxidises Fe2- to Fe3+ ?
Answer the following question.
When MnO2 is fused with KOH in the presence of KNO3 as an oxidizing agent, it gives a dark green compound (A). Compound (A) disproportionates in an acidic solution to give a purple compound (B). An alkaline solution of compound (B) oxidizes KI to compound (C) whereas an acidified solution of compound (B) oxidizes KI to (D). Identify (A), (B), (C), and (D).
Using IUPAC norms write the formulae of Potassium trioxalatochromate (III)
Zinc carbonate is precipitated from zinc sulphate solution by the addition of ___________.
On addition of small amount of \[\ce{KMnO4}\] to concentrated \[\ce{H2SO4}\], a green oily compound is obtained which is highly explosive in nature. Identify the compound from the following.
Which of the following is amphoteric oxide?
\[\ce{Mn2O7, CrO3, Cr2O3, CrO, V2O5, V2O4}\]
When \[\ce{Cu^2+}\] ion is treated with \[\ce{KI}\], a white precipitate is formed. Explain the reaction with the help of chemical equation.
Potassium permanganate acts as an oxidant in neutral, alkaline as well as acidic media. The final products obtained from it in the three conditions are, respectively.
Potassium dichromate when heated with concentrated sulphuric acid and a soluble chloride, gives brown-red vapours of ______.
In the two tetrahedral structures of dichromate ion, ______.
Complete the reaction mentioning all the products formed:
\[\ce{2KMnO4 ->[\Delta]}\]
